The invention relates to the measurement of the flow of fluid through a well, especially a hydrocarbon well, so as to provide a flow profile of the well.
To measure the flow of fluid in a hydrocarbon producing well, a flowmeter is lowered into the well at the end of an electric cable which cable allows electrical transmission between the flowmeter and surface equipment.
Flowmeters presently used for this type of measurement are essentially propeller-type flowmeters (French Pat. No. 2 036 200). These well-known apparatus exhibit a number of drawbacks.
From the viewpoint of their principle, it should be noted that the frequency-flow characteristic depends on the properties of the fluid (viscosity, density), thus complicating the calibration of the apparatus.
From the practical viewpoint, the fact that the propeller is supported by bearings is a source of difficulty because bearings are subject to wear and to jamming by sand which may be carried by the fluid. At high flowrates, the propeller undergoes a high thrust which results in accelerated bearing wear. This wear is a major factor of inacurracy at low flowrates. Moreover, owing to solid friction at the bearings, the propeller does not begin to turn until after a given flowrate threshold is passed. Consequently, in a frequency-flow plot, the characteristic curve of the flowrate does not go through the origin of the coordinates.
A need was accordingly felt for the development of a flowmeter adapted to be used in a well, that is, capable of being moved in the well by means of a cable and of being made with a very small cross-section, and on the other hand, free from the above-described drawbacks, that is, having no moving part subject to wear and being easy to calibrate.
The vortex shedding flowmeters disclosed for instance in an article by R. S. Medlock "The Vortex Flowmeter--Its Development and Characteristics" published in Australian Journal of Instrumentation and Control, April 1976, pages 31-32, fulfill the second set of the requirements indicated above.
In these apparatus, an elongated vortex-generating obstacle is placed transversely with respect to the flow conduit with its ends secured to the conduit. The vortices are generated alternately on each side of the obstacle with a frequency proportional to the local velocity of the fluid and hence to the flowrate, forming what are called Karman vortex streets whose spatial period and front width depend only on the geometry of the obstacle. By associating with the obstacle a transducer sensitive to the passage of the vortices, an electric signal is obtained whose frequency is proportional to the flowrate.
Flowmeters of this type exhibit, over a wide range of flowrates, a frequency-flow characteristic of excellent linearity and practically independent of fluid density and viscosity. This characteristic moreover is not liable to be affected after repeated use, because of the absence of moving parts.
However, the fact that in these conventional apparatus, the obstacle is fixed to the conduit by its ends, precludes from contemplating their use in a well as any connection whatsoever between the obstacle and the wall of the well is obviously incompatible with such a use.
There are commerically available vortex shedding flowmeters adapted for lateral insertion inside a conduit marketed by the Eastech Company of Edison, New Jersey, U.S.A., under the references DS-2620 and DS-2630. In these apparatus, the obstacle crosses the interior of a tube diametrically, and means are provided for inserting and fixing the tube inside the conduit so that its axis is directed along the axis of the conduit, the obstacle thus being placed transversely with respect to the flow.
This configuration leads to the separation of the flow actually measured from the main flow owing to the "chimney" effect due to the tube. The secondary flow thus created is often not highly representative of the main flow as concerns the velocity profile, thus distorting the measurements.
Furthermore, beginning at the tube entry, is produced a turbulence capable of disturbing the creation of vortices due to the obstacle.
Japanese patent application No. 51-60104 discloses a comparable device for measuring the air flow in internal combustion engines.
French Pat. No. 2 359 427 describes a device for measuring the flow velocity of a fluid in a free medium, especially for measuring the velocity of a helicopter. This device comprises two parallel obstacles disposed transversely with respect to a flow channel. It is disclosed that the section of this channel may be of any shape, however its dimension in the direction parallel to the obstacles is preferably larger than its dimension in the direction normal to the obstacles. This flow channel can be equated to the tube of the above-cited DS-2620 and DS-2630 flowmeters.
U.S. Pat. No. 3,810,388 indicates that in the case of rectangular section conduits, the obstacles must have an efficient length smaller than the corresponding dimension of the conduit so that the ratio of the length to the front width of the obstacle does not exceed a given limit thus allowing a satisfactory vorticity. Provision is then made to equip the obstacle with end plates designed to prevent the fluid from flowing along the obstacle.
Further, French Pat. No. 2 444 260 indicates that it is advantageous to create a bidimensional flow around the obstacle by using a rectangular or substantially rectangular flow section. A prior art arrangement in which the obstacle has its ends respectively attached to two parallel walls secured in turn to the interior of the conduit is indicated as being disadvantageous, and this patent teaches an arrangement in which the obstacle has its ends secured to the conduit and walls parallel to the axial direction of the obstacle are disposed on both sides of the latter.